Natural gas compressors receive natural gas from wells and compress the gas into compressed natural gas (“CNG”), which is more readily stored. Lubrication systems for natural gas compressors are described, for example, in U.S. Pat. No. 5,835,372 to Roys et al. To lubricate the compressor, a small volume of lubricant at high pressure is typically applied periodically into the compressor cylinder. A check valve inserted between the lubricant line and the compressor cylinder seals the compressor cylinder to prevent natural gas from flowing into the lubrication line. The high pressure lubricant periodically opens the valve to inject the lubricant. The lubricant is at a higher pressure than the natural gas, so when the valve is open, the lubricant flows into the cylinder, instead of the natural gas flowing out of the cylinder. To prevent the hot natural gas from contaminating or otherwise damaging the check valve, an oil reservoir device is typically mounted between the cylinder and the check valve to maintain an oil barrier between the check valve and the natural gas.
FIG. 1 shows a cross section of a typical reservoir device 100 mounted on a compressor 102 to provide an oil barrier between cylinder 104 and check valve 106. In operation, reservoir device 100 is filled with oil to the top of a tube 110 to ensure an oil barrier between check valve 106 and cylinder 104. The check valve 106 is typically connected to the reservoir using a nipple 116 having pipe threads 118 on both ends. The reservoir 100 is typically screwed into the top of the compressor 102 using NPT (national pipe tapered) pipe threads 120 and opens into cylinder 104. For many years, the compressor industry has been replacing failed compressor check valves without understanding the cause of the failure. Applicant has determined the cause of many check valve failures and the method and apparatus described below reduces check valve failures.